There has heretofore been known such an injection molding machine as disclosed, for example, in Japanese Patent Publication No. 57-59060 which is arranged to inject and charge the molten resin in a mold cavity from an injection nozzle by forwardly moving an injection screw mounted so as to be rotatively driven and reciprocated freely in a housing by the action of an injection cylinder located behind the base end of the screw.
Generally, in injection molding machines, it is necessary to control the injection speed of the molten resin to be charged into the mold cavity depending on the position of the injection screw.
The injection speed in the above-mentioned conventional injection molding machine is determined by the travel speed of the injection screw, that is, the speed of expansion of the injection cylinder, and therefore, in order to control the injection speed, there is no alternative to controlling the volume of hydraulic oil to be supplied into the expansible pressure chamber in the injection cylinder depending on the position of the injection screw to vary the speed of expansion of the injection cylinder as disclosed, for example, in Japanese Patent Application Laid-Open Specification No. 59-64337.
However, since there is a time-lag between the time the speed of expansion of the injection cylinder is changed and the time when the speed and flow rate of the molten resin passing through the injection nozzle and flowing into the mold cavity are varied, the injection speed cannot be controlled rapidly and with a high accuracy depending on the position of the injection screw.
Stated in more detail, since the molten resin itself has compressive properties, the travel speed of the injection screw does not always correspond to the injection speed of the molten resin from the injection nozzle. Therefore, there are instances where even though the speed of travel of the injection screw is controlled with a high accuracy, the accuracy of controlling the injection speed cannot be improved and such a control as to obtain a predetermined injection speed corresponding to the position of the injection screw cannot be achieved.
That is, when the injection screw forces the molten resin during, for example, the resin charging process, the pressure of the molten resin within the housing formed in the leading end of the injection screw is raised. Even if the forward movement of the injection screw is stopped in this condition, the compressed molten resin within the housing flows out from the injection nozzle into the mold cavity until its pressure has reduced to the level corresponding to the loading pressure within the mold, and as a result, there occurs some delay in response until the injection speed becomes zero.
Further, in case of rapidly accelerating the speed of travel of the injection screw from the condition wherein said speed has been changed over to thereby rapidly accelerate the injection speed, it is necessary to obtain momentarily the injection speed corresponding to the speed of travel of the injection screw. But, in order to meet this requirement, the pressure of the molten resin in the housing needs also to be increased correspondingly. As a matter of fact, however, there is some time lag until the pressure of the molten resin increases to such a level, and as a result, there is a corresponding delay in response until the injection speed is increased.
Stated concretely, in the case where a product whose shape varies steeply at two places as shown, for example, in FIG. 1 is produced by injection molding, in order to prevent the occurrence of surface imperfection such as haze weld lines etc. due to turbulence in the flow of the molten resin into the mold cavity, it is necessary to obtain an injection speed pattern as shown in FIG. 2 where four stages of injection speed V.sub.1, V.sub.2, V.sub.3 and V.sub.4 corresponding, respectively to four positions A, B, C and D of the injection screw as shown. V.sub.1 and V.sub.2 are set at a low speed, and in order not to produce any flow mark, V.sub.2 and V.sub.4 are set at a high speed.
In order to control the injection speed in this manner, after detecting the positions A, B, C and D of the injection screw, one set of flow control valves installed in the hydraulic circuit for the injection cylinder is manipulated to change the speed of expansion of the injection cylinder to thereby control the speed of travel of the injection screw as shown by broken line in FIG. 2. But, because of the aforementioned delay in response, the speed of travel of the injection screw is different from the injection speed shown by solid line in FIG. 2 in terms of changing zones in their respective rising and falling zones.
Because of the above-mentioned delay in response of the injection speed relative to the speed of travel of the injection screw, even if the speed of travel of the injection screw is rapidly changed, the injection speed is switched slowly from V.sub.1 to V.sub.2, and from V.sub.3 to V.sub.4 thereby forming flow marks on the surface of the product. Also, during the change of speed from V.sub.2 to V.sub.3 at the time of falling, even if the injection speed is rapidly lowered, the injection speed remains high until the pressure of the molten resin becomes equal to the loading pressure within the mold cavity, and then it is changed over slowly from V.sub.2 to V.sub.3 thus causing haze weld lines on the product. To prevent this phenomenon, if the speed of travel of the injection screw is changed over at a position just this side of the position B of the injection screw at the time of change of the speed from V.sub.2 to V.sub.3, then a flow mark is produced.
Further, even if the speed of travel of the injection screw is reduced before the application of a predetermined pressure; that is to say, the application of a dwell pressure to the extending pressure chamber of the injection cylinder after the completion of charging of the molten resin in order to prevent the occurrence of a molding sink due to the solidification of the molten resin, a peak value V' will occur by the inertia due to the loading on the motor and so forth which is exerted on the injection screw thus forming a flash or flashes on the product.
One of the examples of the conventional devices for controlling dwell pressure application is also disclosed in the above-mentioned Japanese Patent Application Laid-open Specification No. 59-64336.
According to this device for controlling dwell pressure application, the dwell force to be applied to the mold is changed over by forwardly moving the injection screw and charging the molten resin successively into the injection mold at a predetermined pressure while the injection pressure is being changed over stepwise depending on the position of the injection screw, detecting the position of the injection screw by means of a position detector after the completion of the filling and thereby decelerating the speed of forward travel of the injection screw and at the same time reducing stepwise the pressure to be applied to the injection cylinder connected to the injection screw by means of a timer.
In such a device for controlling dwell pressure application, even though the movement of the injection screw is stopped and the pressure in the expansion pressure chamber of the injection cylinder is changed over to a preset pressure in order to keep the injection molding machine under dwell condition, if compressed molten resin remains in the leading end of the injection screw, the pressure of the resin cannot reach the preset dwell force momentarily by the injection force of a motor and so forth applied to the injection screw. In other words, the pressure of the molten resin reaches the preset value with short time lag, and also a peak pressure occurs in the beginning of change-over to the dwell pressure application process thus causing charging of an excess amount of molten resin into the mold cavity, the so-called, overchange.
As a result, an excessive dwell pressure force is exerted on the molten resin prior to the solidification thereof thus forming flashes on the molded product at locations thereof which correspond to the terminal ends and round halves of the mold to be charged with the molten resin.
In order to prevent the occurrence of the above-mentioned initial peak pressure, it is envisaged to put forward the position of the injection screw at the time of switch-over of the dwell pressure. However, by so doing, the change-over to the dwell pressure application process occurs while the mold cavity is not charged completely with the molten resin. As a result, the rate of flow of the resin to be charged into the terminal ends of the mold will drop so that the product is liable to have flow marks formed on the surface thereof.
Further, when the dwell pressure is changed stepwise after the pressure of the molten resin has been changed over to the dwell condition, a time lag occurs between the time of rising and that of falling, and the pressure gradient at the time of rising and falling cannot be controlled. For this reason, there are cases where a steep pressure gradient is required depending on the shape of the product to be obtained, whilst in some cases, a slow pressure gradient is required. However, since such control cannot be effected, molded products sometimes have flashes and/or flow marks.